usb: Add correct depends for CMD_USB_MASS_STORAGE

We cannot build this without USB_GADGET_DOWNLOAD support enabled, add
the appropriate depends line.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index a9fb4ee..f196e6c 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1345,6 +1345,7 @@
 
 config CMD_USB_MASS_STORAGE
 	bool "UMS usb mass storage"
+	depends on USB_GADGET_DOWNLOAD
 	select USB_FUNCTION_MASS_STORAGE
 	depends on BLK && USB_GADGET
 	help